First of all "Hi" to all from a new guy on this forum.
For the long-term exploration group I would go with a Nebula as the core ship. It's a solid explorer in it's own right and you could use the equipment module to hold some runabouts or additional supplies. Even with replicators you would need supplies and spare parts to keep 3 or 4 ships running on a prolonged mission. If you don't want to use a dedicated supply ship those would have to go somewhere.
A Nova class sounds like a good idea at first, but you have to consider that these are not very fast and certainly not build for missions lasting longer than a few months. If you absolutly want to include a Nova pair it with an escort. That way you could "park" the science ship somewhere (long term planetary survey, etc.) an still give it some protection.
Personaly I'd use a light cruiser instead with some extra scientific equipment and laboratories added.
A Norway class ship would be on my list as well. It's fast enough and could hold it's own in a fight, should the need arise.
Add a second Nebula or maybe an Excelsior and you should do fine.
Whatever combination you prefer IMHO you should go with ships that could work and survive independently from each other, should the group be seperated for any reasons.
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"To seek, to strive, to find and not to yield" - Alfred Tennyson
[This message has been edited by Lancer (edited 04-01-2001).]